As a glass composition employed for glass substrates for information display devices, particularly for active matrix liquid crystal display devices, a non-alkali borosilicate glass composition has been used. A typical example of the non-alkali borosilicate glass may be the code 7059 glass of Corning Inc. in the United States.
In recent years, such information display devices were required to be larger in size, and the employed glass substrates became larger in area as well. If defects, such as bubbles, exist in the glass substrates used for manufacturing the information display devices, it causes to reduce the manufacturing yield significantly.
To prevent bubbles from remaining in a glass article during processes of manufacturing glass articles is known as refinement. A method of adding a refining agent to a glass melt for refining the melt is generally known. Arsenic oxide, antimony oxide and fluoride are widely known as the refining agents. A method for employing tin, lanthanoid or a compound of them as a substitute for known refining agents is disclosed in JP2003-192377 A, for example.
On the other hand, one of the refinement methods without a refining agent is vacuum degassing. In this method, a glass melt is kept under a decompressed atmosphere. Thus, the gas dissolved in the melt easily forms bubbles. The bubbles in the melt are expanded by decompression to enhance their buoyancy, and they rise to disappear on a melt surface. The vacuum degassing is a method to refine by these two actions. An example of the vacuum degassing is disclosed in JP2003-160340 A.
Another refinement method without a refining agent is disclosed in JP2003-300750 A. The method disclosed in this publication is a method of refining by including gas, such as helium and neon, in the molten glass composition.
However, glass compositions for substrates of information display devices, such as a non-alkali borosilicate glass composition, have a characteristic of high viscosity as a melt (high viscosity at high temperatures). Since degassing and refining a melt with a high viscosity is not easy, it is fundamentally difficult to reduce defects such as bubbles in the glass substrate using these glass compositions.
In order to degas and refine such glass compositions with a high viscosity, arsenic oxide, antimony oxide or fluoride mentioned above are employed for a refining agent. However, the heavy load to the environment when using arsenic oxide has been pointed out with the increasing awareness for the environment in recent years. Accordingly, using these known refining agents should be avoided as much as possible.
In many of the Examples disclosed in JP2003-192377 A, Sb2O3, which is a known refining agent, is included. That is, it can be concluded that substituting all the alternative refining agents disclosed in this publication for known refining agent is difficult.
The vacuum degassing disclosed in JP2003-160340 A has problems, such as the ones below:                a melting furnace having a complicated structure with a difference of elevation is required;        sealing maintenance at high melting temperatures is required;        lining with expensive platinum materials is required;        specific structures and operations are required to fill the joints of the refractory;        frequent maintenances are required to maintain the specific structure; and        the operation rate of the melting furnace is low.        
The method of including gas in the molten glass, disclosed in JP2003-300750 A, does not fit into mass production. This is because the gas used for this method is expensive and the manufacturing costs become high when used in a large amount for mass production.
The following properties are required to glass compositions employed for the substrates of information display devices.
(1) To have an average linear thermal expansion coefficient similar to that of silicon materials, since silicon materials are employed for transistors for active matrix liquid crystal display devices. The average linear thermal expansion coefficient of silicon is about 32×10−7° C.−1.
(2) To be able to endure high temperatures, since the glass substrates are exposed to heat treatments during the processes of manufacturing transistors. It is necessary that the glass compositions do not deform, melt and devitrify during the heat treatment.
(3) To be unaffected by treatment fluid for processing. The glass compositions should not inadequately melt by and should not become frosted from chemicals such as fluoric acid used in the processes of manufacturing transistors.
(4) To elute a less amount of alkali components. A large amount of alkali components is dispersed from the glass substrates to the transistors during the manufacturing processes when using glass substrates employing glass compositions eluting a large amount of alkali components. The transistors are deteriorated by the dispersion of the large amount of alkali, which results in impairment of the performance of information display devices. Among the alkali components, the sodium component in particular affects the devices largely.